Conventionally, there is known an oil supply device for supplying oil to various parts of an engine.
Japanese Patent No. 5,168,372 discloses a technique, in which the amount of oil to be supplied to bearing portions such as a crank bearing is regulated by a flow rate adjustment valve in such a manner that the oil pressure of a variable valve train mechanism reaches a target pressure or more when an engine is started to secure operability of the variable valve train mechanism from start of the engine.
Japanese Unexamined Patent Publication No. 2014-199011 discloses a technique, in which a plurality of hydraulically operated devices (hydraulic VVT mechanisms and valve stop devices) are provided, a maximum required hydraulic pressure out of the required hydraulic pressures of the hydraulically operated devices is set as a target hydraulic pressure according to an operating condition of an engine, and the discharge amount of a variable capacity oil pump is controlled by an oil control valve in such a manner that an actual hydraulic pressure coincides with the set target hydraulic pressure.
When an engine is started in a cold state, an oil pump is operated in a state that the oil viscosity is high. Therefore, oil may not be supplied to the entirety of an oil supply passage. As a result, a situation occurs such that oil is sufficiently supplied to a main gallery where a hydraulic pressure sensor is provided, and an excessively large actual hydraulic pressure is detected by the hydraulic pressure sensor regardless that oil is not sufficiently supplied to a pressure chamber of an oil pump provided on an oil supply passage downstream of the main gallery.
In this case, even when an oil control valve is controlled in such a manner as to lower the actual hydraulic pressure to a target hydraulic pressure by feedback control, oil may not be sufficiently supplied to the pressure chamber. As a result, the oil pump may fail to adjust the discharge amount of oil, and may be incapable of lowering the actual hydraulic pressure. Consequently, the control amount of the oil pump for lowering the actual hydraulic pressure may further increase.
Eventually, a sufficient amount of oil is supplied to the pressure chamber, and the oil pump is brought to a state in which the discharge amount is adjustable. However, in this case, the control amount of the oil pump for lowering the actual hydraulic pressure is excessively large. As a result, the actual hydraulic pressure may be sharply lowered below the target hydraulic pressure, and the discharge amount of the oil pump may excessively decrease.
The amount of oil to be supplied to the pressure chamber depends on the discharge amount of the oil pump. Therefore, when the discharge amount of the oil pump excessively decreases, a sufficient amount of oil may not be supplied to the pressure chamber, and the oil pump may be brought to a state in which the discharge amount is not adjustable. In this case, even when the oil control valve is controlled to raise the excessively low actual hydraulic pressure to the target hydraulic pressure, the actual hydraulic pressure may not easily rise to the target hydraulic pressure. As a result, the control amount of the oil pump for raising the actual hydraulic pressure may increase. Eventually, the oil pump is brought to a state in which the discharge amount is adjustable. However, in this case, the control amount of the oil pump for raising the actual hydraulic pressure is excessively large. Therefore, the actual hydraulic pressure may sharply increase.
As described above, when feedback control is applied when an engine is started in a cold state, hunting by the actual hydraulic pressure may increase, and a long time may be required until the actual hydraulic pressure reaches the target hydraulic pressure. As a result, it may be impossible to perform stable control of a hydraulically operated device such as a hydraulic VVT mechanism in an early stage.
Further, both of Japanese Patent No. 5,168,372 and Japanese Unexamined Patent Publication No. 2014-199011 are made based on the premise that feedback control is performed. Therefore, it may be impossible to solve the aforementioned drawbacks when an engine is started in a cold state.